Recently, nonvolatile memories capable of storing a large volume of data at a high speed have been developed along with the development of digital technologies.
A flash memory and a ferroelectric memory are well-known as such nonvolatile memories.
Among these nonvolatile memories, the flash memory includes a floating gate embedded in a gate insulating film of an insulated-gate field-effect transistor (IGFET), and stores information by accumulating electric charges indicating recording information, in the floating gate. However, there is a drawback that a relatively high voltage needs to be provided to such a flash memory since it is necessary to flow a tunnel current to the gate insulating film of the flash memory at the time of writing and erasing the information.
In contrast, the ferroelectric memory, which is also referred to as a ferroelectric random access memory (FeRAM), stores information by utilizing the hysteresis characteristic of a ferroelectric film provided to a ferroelectric capacitor. The ferroelectric film causes polarization in response to a voltage applied between upper and lower electrodes of the capacitor, and spontaneous polarization remains even after the voltage is removed. When the polarity of the applied voltage is reversed, the polarity of the spontaneous polarization is also reversed. By causing directions of the polarity to correspond respectively to “1” and “0,” the information is written in the ferroelectric film. Advantages of the FeRAM are that the voltage required for writing in the FeRAM is lower than that required for writing in the flash memory, and that it is possible to write information in the FeRAM at a higher speed than that of the flash memory. A system on chip (SOC), on which a FeRAM and a logic circuit are mixedly mounted, has been examined to be used for an IC card and the like by utilizing the above advantages.
A capacitor dielectric film provided to the ferroelectric capacitor is formed of, for example, a PZT (Lead Zirconate Titanate: PbZrTiO3) film. There are various kinds of methods for forming the capacitor dielectric film.
For example, in Japanese Patent Application Laid-open Publication No. Hei 11-292626, the PZT film is formed by a sol-gel method using a solution in which an organometallic compound is dissolved in an organic solvent such as butanol. The sol-gel method has an advantage that costs for forming a film is lower than those in a case of a sputtering method, a MOCVD method or the like. Hence, the sol-gel method has been widely studied and developed.
On the other hand, as described in APPL. Phys. Lett. 65, P. 1522 (1994), iridium oxide films are often formed as the upper and lower electrodes of the ferroelectric capacitor. According to APPL. Phys. Lett. 65, P. 1522 (1994), in the case where the PZT films are used as the capacitor dielectric films, fatigue of the ferroelectric capacitor can be suppressed, and thus preferable capacitance characteristics can be secured, by forming the upper and lower electrodes of iridium oxide.
However, it is known that huge crystals formed of abnormally-grown iridium oxide are easily generated on a surface of an iridium oxide film. The huge crystals deteriorate electric characteristics of the ferroelectric capacitor, and this may finally cause a decrease in yield of semiconductor devices.
To solve such a problem, in Japanese Patent Application Laid-open Publication No. 2001-127262, a two-step sputtering method is used to suppress generation of the aforementioned huge crystals. Two-step sputtering method includes the first step of forming a film with low sputtering power, and the second step of growing the film with high sputtering power, and these two steps are sequentially carried out. (paragraph 0025).
In Japanese Patent Application Laid-open Publication No. 2000-91270 (JP No. 2000-91270 A), a laminated film which is configured by forming an iridium oxide film and an iridium film in this order, is used as an upper electrode. According to JP No. 2000-91270 A, the iridium oxide film of the lower layer prevents deterioration of capacitance characteristics, and the iridium film of the upper layer reduces resistance of the upper electrode (paragraph 0027).
In Japanese Patent Application Laid-open Publication No. 2002-246564, a PZT film formed by a sputtering method is crystallized by performing the first annealing for the PZT film (paragraph 0035). Then, after an upper electrode made of iridium oxide is formed on the PZT film, the second annealing is carried out on the upper electrode (paragraph 0038).
In Japanese Patent Application Laid-open Publication No. 2005-183842, a laminated film formed of first and second conductive metal oxide films, both of which are formed of iridium oxide, is used as an upper electrode (paragraph 0035 to 0037).
Similarly, in Japanese Patent Application Laid-open Publication No. 2006-73648, a two-layered iridium oxide film is formed as an upper electrode (paragraph 0033).